This invention relates to a composition and method for the surface treatment of copper, and more particularly to a composition and method for improving the surface adhesion properties of copper foil. Using the method and composition of the invention the adhesion of subsequently applied coatings to the copper surface, as for instance during the manufacture of printed circuit boards, is improved.
In the manufacture of printed circuit boards (PCBs) it is generally necessary for a copper foil to be bonded to an insulating polymeric substrate. The copper foil is then imaged and etched to form the circuit layout. In the assembly of multilayer PCBs several such copper-substrate layers, referred to as "inner layers", are bonded together to form a laminate.
In such a manufacturing process it is essential that the layers making up the PCB laminate should bond effectively together. To this end, it is known for the copper surface to be treated with an adhesion promotion composition which roughens, on a microscopic scale, the smooth surface of the copper, thereby improving the adhesion characteristics of the copper surface for subsequently applied layers of the laminate.
U.S. Pat. No 5,037,482, for instance, discloses the use of cleaning and adhesion promotion compositions comprising an aqueous solution of an alkane sulfonic acid, a surfactant and an oxidising agent such as ferric nitrate.
In another approach, described in International Patent Application No WO96/19097, an adhesion promotion composition comprises an aqueous solution of hydrogen peroxide, an inorganic acid, a corrosion inhibitor and a surfactant, most preferably a cationic surfactant such as a quaternary ammonium compound.
Such prior art compositions offer some benefits, but suffer also from a number of disadvantages or potential disadvantages. In particular, the degree of adhesion which may be achieved may not be entirely satisfactory. In addition, the use of surfactants may be undesirable from the point of view of waste disposal. Also, the concentration of the organic components may be insufficient for them to be readily analysed. Such analysis may be desirable for process control purposes, to ensure that the correct concentrations are maintained during use.